Halliday & Matthiessen (2014: 517):
But they also include clauses where the uncertainty is represented grammatically in the projecting clause by a feature of negative polarity or interrogative mood, or by projection or expansion within the verbal group serving as Process, or by perfective aspect in a purpose clause; for example:
||| I do not know || whether you have seen it. |||
||| It is not known || whether the mystics could give objective certainty to their experiences. |||
||| Who knows || whether his debt was true or false? |||
||| So I want to know || whether this devious and hypocritical me could have been whole and innocent at least as a boy. |||